In the oil industry, more precisely in circulating bed hydrocarbon feed conversion processes in the presence of a divided solid with catalytic or non catalytic action, the feed is brought into contact with particles of a divided solid in a substantially vertical reactor, the gaseous phase and the solids leaving the same extremity, either at the top of the reactor is the overall flow is upwards (a riser reactor), or at the bottom thereof when the overall flow is downwards (a dropper reactor). The reaction products must be separated from the catalytic or non catalytic particles at the reactor outlet; the reaction products are entrained towards a secondary separation step and the solid particles are recycled to the reactor inlet, in some processes after regeneration which restores the particle properties (for example coke content, activity) which are suitable for the reaction.